TECHNICAL BACKGROUND OF THE INVENTION
Pillared interlayered smectites (PILCs) with a large variety of pillars have been described in the scientific literature (journals, patents), among which the Al-pillared clays are the most documented ones. Similar materials with pillars based on other elements such as Zr, Cr, Ti, Si, Fe, Ga, Si, Ta, V, Mo, Nb, combinations of two or more of these elements or combinations of one or several of those elements with others elements not mentioned above (as e.g. Ni, Cu, Co, etc.), rare-earth (La, Ce . . . )--containing pillars have been successfully prepared and reported in the literature. Pillared clays containing two or more elements in the pillars are also named mixed pillared clays.
Pillared clays show interesting potentialities in catalysis, as catalysts or supports to catalytic phase(s) or in admixture with other catalysts or catalyst components (e.g. zeolites, metal oxides, etc.), especially as catalysts for e.g. hydrocarbons transformation. Pillared materials also find potential interest as adsorbents and in other domains such as in gas separation processes; as scavengers for heavy metals (treatment of waste water); in SO.sub.2 and NO.sub.x abatement; in purification of edible oil, cation selective composite membranes; as solid electrolytes; as host materials for (conducting) polymers; etc.
Trioctahedral micas
Trioctahedral micas refer to layered 2:1 sheet (or lamellar) silicates in which the octahedral layer is sandwiched between two adjacent tetrahedral layers and mainly contains divalent cations with the results that all the possible octahedral positions are occupied. They differ from dioctahedral micas (muscovite-type), where 2/3 of the octahedral positions are filled with mostly trivalent cations. The general formula of the end-member phlogopite mineral is K.sub.2 Mg.sub.6 (Si.sub.6 Al.sub.2)O.sub.20 (OH,F)4. The structural substitutions mainly occur in the octahedral layers but also in the tetrahedral ones and are responsible for the wide range of chemical compositions of the trioctahedral micas. The high number of substitutions is at the origin of the high net negative layer charge in micas. Potassium is usually the dominant interlayer cation ensuring electroneutrality of the layers. Trioctahedral micas may contain substantial amounts of fluorine (replacing structural hydroxyls) which conveys resistance to weathering, hardness and thermal resistance. The principal cations in the octahedral layer of natural trioctahedral micas are Mg.sup.2+, Fe.sub.2+, Al.sup.3+ and Fe.sup.3+, with smaller proportions of Mn.sup.2+, Ti.sup.4+ and Li.sup.+. Phlogopites refer to trioctahedral micas in which more than 70% of the occupied octahedral sites contain Mg.sup.2+, whereas biotites define the micas where 20 to 60% of these sites are Mg.sup.2+ [Newman & Brown, in Chemistry of Clays and Clay Minerals, A. C. D Newman (Ed.), Mineralogical Soc. 6, Longman, 1987, p. 75]. The potassium ions located between the unit layers just fit into hexagonal cavities (perforations) in the oxygen plane of the tetrahedral layers. Adjacent layers are stacked in such a way that the potassium ion is equidistant from 12 oxygens, 6 of each tetahedral layer [R. E. Grim, Clay Mineralogy, McGraw-Hill, 1953, p.65]. In their original state, natural micas do not swell in the presence of water or polar solvents because the hydration energy of the interlayer potassium ions is insufficient to overcome the co-operative structural forces at the coherent edges of a cleavage surface [Newman & Brown, Nature 223, 175, 1969].
The absence of swelling properties of natural micas makes it impossible, without modifying the mineral, to obtain pillared intercalated forms equivalent to those readily obtained with swelling clays (smectites) in which the clay sheets are separated from each other by pillars of inorganic nature, which confer to these materials thermally resistant structural and textural characteristics such as permanent elevated spacings, high specific surface area and micropore volume, and surface properties (acido-basic, redox).
Vermiculites
Vermiculites belong to a group of hydrated aluminium silicates. These minerals may be considered as "swelling trioctahedral micas" containing Al-for-Si substitutions in the tetrahedral layers (as in micas), and Al-, Fe-, and Ti-for-Mg substitutions in the octahedral layers. Because of both types of substitutions, the overall negative charge of the structure results, as in micas, from an imbalance between the negative charge of the tetrahedral layer and the excess positive charge of the octahedral layer. As in micas and smectites, the excess negative charge is counterbalanced by cations located in the region between adjacent sheets which ensure electroneutrality of the layers. Most often, the interlayer cations are magnesium ions. The layer charge densities in vermiculites are intermediate between those of micas and smectites. Unlike micas, vermiculites may swell and the layers may expand when polar molecules are introduced in the interlamellar region but this swelling capability is much reduced compared with smectites. The interlayer charge balancing cations (magnesium ions) are exchangeable.
Vermiculites (and a fortiori micas) could not be intercalated with bulky poly-hydroxy-aluminum species to form a pillared material exhibiting spacings of about 17-18 .ANG. (gallery height of about 8 .ANG.) as in pillared smectites, a failure which has been attributed to the high layer charge density of these minerals. Contacting vermiculite suspensions with Al.sub.13 -containing pillaring solutions led to expanded materials exhibiting only about 14 .ANG. spacings [references 1-7]. Taking advantage of the high spacings (27-28 .ANG.) developed upon adsorption of long chain amines and alcohols to introduce Al pillars was unsuccessful [reference 5]. Preliminary dealumination of vermiculite by treatment with an aqueous solution of (NH.sub.4).sub.2 SiF.sub.6 followed by the addition of the pillaring solution did not result in materials with improved spacings [reference 7]. A mixture of a pillared fraction of vermiculite (with 18 .ANG. spacing stable at 500.degree. C.) and of unpillared fraction was obtained upon contacting with Al.sub.13 -containing solutions a suspension of vermiculite that was previously treated with L-ornithine [reference 8]. However, repeated attempts to reproduce the method were unsuccessful.
State of the Art
The documents U.S. Pat Nos. 5,200,378 and 5,017,537 are concerned with the pillaring of synthetic layered phosphates. Layered phosphates have nothing in common with natural micas. The intercalation is performed after a previous intercalation of an amine (amide or dimethyl sulfoxide) in order to expand the interlayers. Attempts to pre-swell vermiculite with a long chain amine or alcohol and to treat the expanded vermiculite with a pillaring solution did not allow to obtain 18 .ANG. Al-pillared vermiculite.
The documents U.S. Pat. No. 5,340,657 and EP-0240359 deal with the Al-pillaring of synthetic sodium tetrasilicic fluor micas which have nothing in common with natural micas. The Na-TSF micas have only octahedral substitutions (Li for Mg or Mg for Al), but no aluminium in the tetrahedral layers. Natural micas have substitutions in both the tetrahedral (Al for Si) and octahedral (Al, Fe for Mg) layers. Na-TSF micas are synthesized in a soda-containing medium (thus no interlayer potassium as in natural micas). The presence of exchangeable Na in the interlayers as charge neutralizing cations confers swelling properties. Natural micas have potassium ions between the layers and do not swell in polar media. Na-TSF micas can be pillared when they are contacted with the pillaring solution. Nothing like occurs when doing so with natural micas. This is the principal reason for the prerequisited conditioning operation of the natural micas (aiming at the charge reduction of vermiculites and micas and conversion to homoionic form of hydrated ions). Synthetic Na-micas have, as hydrothermally synthetic layer materials, very small particle sizes. Particles of the order of 0.1 micron are preferred in the document EP-0240359 (p. 3, lines 8-10).
The document U.S. Pat. No. 4,510,257 describes a method which allows to intercalate three-dimensional silicon oxide pillars from organo-silicon derivatives in the clay interlayers. The material is then calcined to decompose the organic moiety. Vermiculite is mentioned (yet no example of successful Si-pillared material is provided).
The document WO98/00091 deals with the pillaring of synthetic layered silicate materials which have no octahedral layers and are thus different from either synthetic sodium fluor tetrasilicic "micas" or natural micas (as in our patent application), both of which having octahedral layers.